Lyme disease, which is caused by the tick-borne spirochete Borrelia burgdorferi, can usually be treated successfully with 2-4 week courses of oral or intravenous antibiotic therapy. However, approximately 10% of patients with Lyme arthritis have persistent synovial inflammation despite such courses of therapy. To explain antibiotic-refractory Lyme arthritis after 2-3 month courses of oral and IV antibiotics, the competing hypotheses of persistent infection or infection-induced autoimmunity have been proposed. In the absence of an animal model, clinical correlations in human patients are required to address these hypotheses. To date, most patients with antibiotic-refractory Lyme arthritis have had negative PCR results for B. burgdorferi DNA in joint fluid after antibiotic treatment;they have had HLA-DRB molecules that bind the OspA163-i75 epitope of the spirochete, particularly the DRB1*0401, 0404 and 0101 molecules, and T cell reactivity with this epitope. Our goals in the next grant cycle include an assessment of antibody responses to lipid and carbohydrate antigens of B. burgdorferi to determine whether a decline in one of these responses after antibiotic therapy might serve as a practical surrogate marker for spirochetal killing in Lyme disease. The precursor frequencies of T cells specific for OspA163-i75 or implicated autoantigens will be determined in both the infectious and post-antibiotic periods of the illness, the clonal characteristics of these cells will be delineated, and the numbers and function of T regulatory cells will be determined during active arthritis through arthritis resolution. To broaden the search for candidate autoantigens beyond those identified by sequence homology with the OspA-,63-175 epitope, B cell epitopes of putative autoantigens will be sought using patient sera to screen a large array of expressed human proteins, and T cell epitopes will be sought using patient synovial tissue as a source of naturally processed MHC complexes from which peptides will be eluted and identified using tandem mass spectrometry. The knowledge gained from these studies is likely to aid in more accurate diagnosis and more effective treatment of antibiotic-refractory Lyme arthritis. Of greater general importance, antibiotic-refractory Lyme arthritis, which shares similar HLA associations with rheumatoid arthritis, may serve as a model of infection-induced autoimmunity. Furthermore, this work brings us closer to the long-term goal of direct comparisons of antigens in Lyme arthritis and rheumatoid arthritis synovia.